<!doctype html>
  <header>
    <style>
      #output {
        width: 400px;
        height: 350px;
        font-family: Lucida Console, Courier, monospace;
        display: block;
      }
      #input {
        width: 400px;
      }
    </style>
  </header>
  <body>
    <textarea id="output" placeholder="type '--help' and hit [enter] to get started" readonly="readonly"></textarea>
    <input type="text" placeholder=">" id="input" autocomplete="off" />
    <script type="module">
      // Import the ESM version of the yargs module:
      import Yargs from 'https://unpkg.com/yargs@16.0.0-beta.1/browser.mjs';
      const yargs = Yargs()
        .scriptName('>')
        .command('clear', 'clear the output window', () => {}, () => {
          output.value = '';
        })
        .command('alert <message...>', 'display an alert', () => {}, (argv) => {
          alert(argv.message.join(' '))
        })
        .wrap(null)
        .strict()
        .demandCommand(1)
        .version('v1.0.0');

      // Use the #output element to output parsed "cli" #input:
      const input = document.getElementById('input');
      const output = document.getElementById('output');
      input.addEventListener('keydown', event => {
        if (event.key === "Enter" || event.code === "Enter" || event.keyCode === 13) { // Enter.
          const argv = yargs.parse(input.value, (err, argv, _output) => {
            if (output) {
              output.value += `\n${_output}`;
              output.scrollTop = output.scrollHeight;
            }
          });
          input.value = '';
        }
      });
    </script>
  </body>
</html>
